Federal safety officials said the engineer misinterpreted a signal warning him to slow down and alerting him to stop for any trains or obstructions ahead. The train should have been traveling at 15 m.p.h., but the engineer was instead operating the train at 40 m.p.h. when it slammed into the rear of a freight train.

On Tuesday, the National Transportation Safety Board blamed Amtrak for not ensuring the engineer was properly trained. In addition, a relief engineer didn't tell the engineer he had misinterpreted the signal.

"We will continue to improve supervision, competency testing, training, and evaluation for Amtrak engineers, as recommended by the NTSB," Amtrak spokesperson Marc Magliari said, according to the Tribune.